Phase change inkjet printers receive phase change ink as a solid, which may be in the form of solid blocks or ink sticks, pellets, or pastilles. The solid ink is loaded into a printer and then melted to produce liquid ink that is used to form images on print media. Phase change inkjet printers form images using either a direct or an offset (sometimes called indirect) print process. In a direct print process, melted ink is jetted directly onto print media to form images. In an offset print process, also referred to as an indirect print process, melted ink is jetted onto a surface of a rotating member, such as the surface of a rotating drum, belt, or band. Print media are moved proximate the surface of the rotating member in synchronization with the ink images formed on the surface. The print media are then pressed against the surface of the rotating member as the media passes through a nip formed between the rotating member and a transfix roller. The ink images are transferred and affixed to the print media by the pressure in the nip.
Offset phase change inkjet printers utilize drum maintenance units (DMUs) to facilitate the transfer of ink images to the print media. A DMU is usually equipped with a reservoir that contains a fixed supply of release agent (e.g., silicone oil), and an applicator for delivering the release agent from the reservoir to the surface of the rotating member. One or more elastomeric metering blades are also used to meter the release agent onto the transfer surface at a desired thickness and to divert excess release agent and un-transferred ink pixels to a reclaim area of the drum maintenance system. The collected release agent is filtered and returned to the reservoir for reuse.
A small amount of release agent is removed from the system with each print. The control system of the printer utilizes a life-sensing process to predict when the supply of release agent is likely to be depleted so an alert can be generated indicating that the DMU is in need of replacement before the supply is exhausted. Volume sensors are impractical so previously known life-sensing processes involve various combinations of open loop print counting and predictions of oil mass remaining in the source following detection of a float sensor reaching a predetermined level in the source. An end-of-life condition is sensed in response to air being detected in the oil intake from the source.
During transfer of the ink images from the image receiving member to the media sheets, each of the media sheets extracts a small quantity of release agent from the surface of the image receiving member. The ink pixels that are placed on the oiled and metered imaging surface typically remove more oil from the imaging surface during transfer than the non-inked media does. This difference in oil removal produces a differential oil “ghost” of the image that resides on the image receiving member until the next drum maintenance cycle. This oil “ghost” is erased by the subsequent flooding of oil on the imaging member by the oil applicator and the metering of the release agent layer by the elastomeric blade. Additionally, since some ink drops in an ink image often fail to transfer to the media sheets, the image receiving member carries some residual ink drops that can transfer to a subsequent media sheet resulting in a “freckling” effect on the subsequent media sheet. To reduce or prevent ghosting and freckling, the DMU applies a new coating of release agent to the image receiving member after each image formation and transfer operation from the image receiving member. The metering blade removes residual ink drops from the image receiving member and forms a layer of release agent with uniform thickness on the image receiving member. Frequent use of the DMU reduces the operational life of the DMU, but is necessary in existing printers to avoid excessive ghosting and freckling. Thus, improvements to printers that lengthen the life of the DMU while maintaining acceptable image quality during printing are desirable.